Synchronous Optical NETwork (SONET) is well-known in telecommunications to offer cost-effective transport of optical communication signals, in terms of both “access” to the network as well as transport between nodes within a telecommunications network. FIG. 1 illustrates how signals are carried in SONET. First, SONET packages a signal into “containers” 1. Section overhead 2 is then added to each container 1 so that both the signal and the quality of transmission are traceable. The containers have two different names depending on size: virtual tributary (VT) or synchronous payload envelope (SPE). The path overhead contains data to control the facility (end to end) such as for path trace, error monitoring, far-end error or virtual container (VC) composition. SONET defines Optical Carrier (OC) levels, as well as electrically equivalent Synchronous Transport Signals (STSs) for the transmission hierarchy. The base format of SONET is a synchronous STS-1 signal that operates at 51.84 Mbps.
The format of an exemplary STS-1 SONET frame 10 is illustrated in FIG. 2. As shown, STS-1 frame 10 includes 90 columns and 9 rows, yielding a frame size of 810 bytes. This equates to a basic STS-1 transfer rate of 51.84 Mb/s. The frame consists of two main areas: (1) transport overhead 12, which occupies the first three columns and (2) the synchronous payload envelope (SPE) 14, occupying the remaining 87 columns. The signal is transmitted byte-by-byte beginning with “byte one” at location 16, scanning left to right from row one to row nine. The entire frame is transmitted in 125 microseconds. Higher level signals (STS-N) are defined as integer multiples of the base rate. STS-1 frames are interleaved and converted to optics to form optical channel OC-N signals.
The performance parameters that the SONET owner measures may include, for example, code violations, errored seconds, and severely errored seconds. An errored second is defined as a one second period during which at least one errored block is transmitted. An errored block is a SONET block having at least one errored bit. A severely errored second is defined as a one second period during which 30% or more of the transmitted blocks are errored. Thus, at the STS-1 rate, an errored second occurs whenever at least one of the 6264 bits in at least one of the 8000 blocks transmitted in one second is errored. Similarly, a severely errored second occurs when t least one bit in each of at least 2400 of the 8000 transmitted blocks in one second is errored.
While monitoring a network outage, SONET technicians need to properly assess/measure the impact of the SONET outage on subscribers. In most cases, SONET outages are defined in terms of “minutes of outage” (MOO). Since the SONET hierarchy consists of various lower level and higher level facilities, it is possible for an “outage” on a lower (for example, T-1) level to be “counted” at the same time as an outage on the OC-48 level. This double-counting, as well as counting “actual” outages during planned outages (for maintenance, for example), has resulted in inaccurate reporting in terms of the impact of SONET outages on subscriber. Thus, a need remains for a technique for accurately defining the outage impact related to failures in the SONET system.